This is going to sound stupid, but here goes:

If I'm watching HDTV via QAM, the audio is "dolby digital" (usually 5.1 except on the commercials where it's 2.0)

I've tried to follow the instructions for getting "passthrough" enabled but it seems like there's 5 diff. ways to do it.

Setup scenario 1:
Can the audio sources be mixed?
Meaning, One AirStar HD card (gets the audio via the RG-6 line), and output via SPDIF, but the 2nd card - PVR-150 with input from Cable STB which is analog? Can you have both output via the SPDIF? I am assuming I do NOT want to connect the STB audio directly to my receiver (otherwise if it's like Tivo you get the audio about 3 seconds before the video because of the hard drive recording delay), but rather connect it to the audio inputs on the sound card (but then I don't think it will output via SPDIF, right?) I need to run a 2nd set of cables from the audio card output to the receiver as well? And just switch between the two?

Setup scenario 2:
ASSUMING the above is true, how does one go about configuring for passthrough? I've read the wiki which seems to indicate that I can do a:

cat /proc/asound/pcm

I see two entries. The first one is the digital, but it only lists playback, not recording.
The 2nd entry is NOT digital and lists playback AND recording

0:1 is the digital (it says I can just "use it" but doesn't say how).

I ASSUMED that to "use" the digital, I go into Setup -> General and specify the device as /dev/adsp

and I checked the box for "enable AC3 to SPDIF passthrough"

But I get no sound to the receiver (via SPDIF).

Do I need to stop/restart for it to take effect?

Am I supposed to type something into the ALSA field like:
ALSA:spdif
?

If I understand your first question correctly, yes SPDIF can be used to give both 2-channel PCM and AC3 audio to your receiver. That's how I have one of mine setup and it works perfectly (except my receiver keeps processing the 2 channel to Dolby ProLogic, but that's a whole different story).

As for your second question, On my setup with a chaintech av710 soundcard all I ever had to do was change the audio output to adsp. I also checked the passthrough boxes, but since I only record SD I don't think that ever comes into play. I don't believe you need to restart to have the settings take effect.

I would try the other options under output and see if any of them work (adsp1...). I'd also run "alsamixer" and make sure your outputs aren't muted. I also had a problem with R5D1 where alsa would detect my onboard sound even though it was off in the bios (go figure), so when you run alsamixer make sure it's controlling the right card. You could also try playing a file from the command line and see if you get anything there.

If none of that helps, you're out of my experience.

I got it working.

Apparently you have to exit ALL the way out of Myth and stop / start the back-end again.

I'd already setup everything as I should (set to /dev/adsp) and enabled the AC3 passthrough, but it wasn't outputting any sound.

I exited out of Myth, stopped the back end, started it back up and went into Myth again and there was the sound.

Yay!